1. Field of the Invention
The present invention relates to a polyvinyl chloride resin composition having an excellent impact resistance, particularly a falling weight impact resistance, stress-whitening resistance, and optical characteristics.
2. Description of the Related Art
Polyvinyl chloride resin has excellent chemical and physical properties, and therefore, is widely used in many fields. Nevertheless, polyvinyl chloride resin per se has a drawback of an inferior impact resistance, and a large number of proposals have been made for an elimination of this drawback.
For example, Japanese Patent Publication (Kokoku) No. 42-20847 proposes a polyvinyl chloride resin composition comprising 30 parts by weight or less of a graft copolymer and 70 parts or more by weight of a polyvinyl chloride resin, wherein said graft copolymer is obtained by graft polymerizing 80 to 30 parts by weight of the monomers as the total of 40 to 80% by weight of a methacrylic acid ester and 60 to 20% by weight of styrene onto 20 to 70 parts by weight of a polymer composed mainly of 1,3-butadiene, said graft polymerization being conducted by first graft polymerizing the methacrylic acid ester and then graft copolymerizing styrene.
Japanese Patent Publication (Kokoku) 47-23648 proposes a polyvinyl chloride resin composition comprising a graft copolymer and a vinyl chloride resin, wherein said graft copolymer is obtained by graft polymerizing first methyl methacrylate in the presence of an elastomer and then graft polymerizing styrene.
Also, Japanese Patent Publication (Kokoku) No. 57-26536 proposes a polyvinyl chloride resin composition comprising 3 to 40 parts by weight of a graft copolymer and 97 to 60 parts by weight of a polyvinyl chloride resin, wherein said graft copolymer is obtained by graft polymerizing 65 to 25 parts by weight of monomer group comprising methyl methacrylate, an alkyl acrylate and styrene and optionally a polyfunctional crosslinking agent in the presence of 35 to 75 parts by weight of a 1,3-butadiene type elastomer, said graft polymerization being conducted by graft polymerizing in the first stage 50 to 90% by weight of the total amount of a major amount of methyl methacrylate and a minor amount of an alkyl acrylate and optionally a polyfunctional crosslinking agent having an allyl group, in the presence of 35 to 75 parts by weight of a 1,3-butadiene type elastomer, and then graft polymerizing styrene in the second stage, and further, graft polymerizing 10 to 50% by weight of the total amount of methyl methacrylate and an alkyl acrylate.
Further, Japanese Patent Publication (Kokoku) No. 2-19145 proposes a polyvinyl chloride resin composition comprising 2 to 40% by weight of a graft copolymer and 60 to 98% by weight of a polyvinyl chloride resin, wherein said graft copolymer is obtained by graft polymerizing 30 to 15 parts by weight of a monomer mixture comprising an alkyl methacrylate and an aromatic vinyl or a monomer mixture comprising an alkyl methacrylate, an aromatic vinyl and an unsaturated nitrile onto 70 to 85 parts by weight of a highly crosslinked butadiene rubber with a particle size of 600 to 3000 .ANG., a total crosslinking agent in an amount of 1 to 10% by weight and a degree of swelling of 7 or less, said graft polymerization being conducted by one stage or multi stage.
In the polyvinyl chloride resin composition proposed in Japanese Patent Publication (Kokoku) No. 42-20847 or 47-23648, however, an ungelled product is liable to remain under extrusion conditions of a low degree of mixing, whereby, for example, a large generation of fish eyes occurs when formed into a sheet, the optical characteristics are poor, the stress-whitening resistance is poor, and the dart falling weight impact strength is too poor. Also, since styrene as the final graft component is polymerized during the preparation of the graft copolymer used, a styrene having a slow polymerization rate often is left, and consequently, remains in the graft copolymer powder and causes an objectionable odor to arise, and thus has a drawback in that the use of such a polyvinyl chloride resin composition is limited.
In the polyvinyl chloride resin composition proposed in Japanese Patent Publication (Kokoku) No. 57-26536, however, although the optical characteristics such as a total light transmittance and haze value are good, a product having a satisfactory stress-whitening resistance and falling weight impact strength cannot be obtained.
Further, in the polyvinyl chloride resin composition proposed in Japanese Patent Publication (Kokoku) No. 2-19145, although the impact resistance is excellent, the optical characteristics such as a total light transmittance and haze value are poor, and further, the stress-whitening resistance is not satisfactory.